The present embodiments relate to a magnetic resonance system.
Coaxial cables that are subject to field fluctuations are used in modern magnetic resonance systems to transport receive signals from local coils. The local coils are used to allow the recording of images with a high signal-to-noise ratio. A local coil or a coil may be an antenna. The coil may include one or more individual coil elements (e.g., loops; an array coil). In addition to the coil elements, the coil may also include a preamplifier, further electronic systems and cabling, a housing and a cable with a plug. The plug may be used to connect the coil to the magnetic resonance system (e.g., to a plug-in station of a patient couch, bed or table).
When recording magnetic resonance images using local coils, excited nuclei induce a voltage in the coil, which is amplified using a low-noise preamplifier (LNA) and forwarded by cable (e.g., at the transmit frequency) to the electronic receive system. To improve the signal-to-noise ratio further, high-field systems may be used. The basic field strengths of the high-field systems may be around 3 Tesla or higher.
When in use, a local coil is located in a transmit field (e.g., B1 field) of a whole body coil. The whole body coil is a large coil that encloses an object to be recorded and is used to excite the spins in the object to be recorded. In this process, the whole body coil generates a circular or elliptical B1 field. This B1 field causes currents to be induced on all the conductors present in the B1 field. Resonance effects may result on longer cables, for example, when the cables act as antennas. In the case of coaxial cables, high currents flow on the outsides of sheath conductors (e.g., shields), which in extreme cases, may become hot and endanger the patient. Before the resulting induced currents are so high that the patient may be at risk, the B1-induced currents may cause secondary B1 fields on the conductor structures of the cables, having an adverse effect on the B1 homogeneity of the transmit field of the whole body coil.
A further problem arises when the coaxial cable experiences a discontinuity (e.g., when the coaxial cable is soldered to a printed circuit board or there is some other departure from coaxial mode). The preamplified receive signals may travel from the inside of the sheath conductor to the outside of the sheath conductor. If the coaxial cable passes close to a coil, the preamplified signal feeds back into the coil, thereby producing self-oscillation of the local coil. In such a state, the local coil may not be used for magnetic resonance imaging.
The currents/waves induced by the transmit field or the receive signals on the sheath conductor of a coaxial cable may be referred to as “sheath waves.” The two problems that may result due to sheath waves were explained above: sheath waves generated by the transmit coil are the cause of B1 homogeneity problems and problems with unacceptably high levels of heat for the patient; and sheath waves generated by the receive signal may feed back into the receive coil and produce self-oscillation of the coil.
To resolve these problems, sheath wave barriers may be provided in line structures that are significantly longer than several tens of centimeters. Sheath wave barriers include resonant traps, which are intended to suppress the current flow at transmit frequency. Since in magnetic resonance systems of the current art, the transmit frequency of the whole body coil and the receive frequency of the local coil and the signal transmitted from the local coils on coaxial lines to the system are identical, sheath waves may only be suppressed on the outside of the sheath conductor. The entire coaxial cable may be wound to an inductance. A parallel capacitor connected to the outside of the sheath conductor completes the sheath wave barrier. The barrier effect results for currents on the outside of the sheath conductor of the coaxial cable. Sheath waves are not suppressed on the inside because the useful signal, which is also at transmit frequency, would also be suppressed. Other types of sheath wave barriers are also known (e.g., bazooka baluns, which are geometrically different in structure but are also intended to only suppress the current on the outside of the sheath and allow the current on the inside to flow unhindered at transmit frequency).
The known sheath wave barriers or baluns have a number of problems.
1. Sheath wave barriers may only be wound manually and are therefore expensive.
2. Because of the large diameter of the coaxial cable and the minimal bending radii of the coaxial cable, sheath wave barriers use a relatively large amount of space (e.g., for the coaxial cable wound to an inductance).
3. The space used in local coils is problematic (e.g., for coils that are to be mechanically flexible).
4. Sheath wave barrier models cannot be matched easily, since the inductance is determined by the number of windings of the coaxial cable inductance. The sheath wave barriers are therefore manually produced special components specifically for the magnetic resonance application.
Arrangements for transmitting magnetic resonance signals have been proposed with a transmission path that connects the local coil to a receiver, with a preprocessing device being provided within the local coil to convert the received signals to at least one transmission frequency that is different from the transmit frequency.
For example, it was proposed in DE 10 2008 023 467 A1 that an arrangement be used, in which two intermediate frequencies are used in order to be able to transmit received signals from a number of channels by way of a single coaxial cable. A first channel of the local coil includes a first coil element for receiving a first magnetic resonance signal and a first mixer connected to the first coil element. The first mixer forms a first signal of intermediate frequency from the supplied first magnetic resonance signal. A second channel of the local coil includes a second coil element and a second mixer, the second mixer forming a second signal of intermediate frequency from the supplied second magnetic resonance signal. The local coil includes a signal combining facility, which combines the intermediate-frequency first signal of the first channel and the intermediate-frequency second signal of the second channel by frequency multiplexing, so that the received signals reach the receiver by way of the transmission path. Two local oscillator frequencies may be used for frequency conversion purposes, each local oscillator frequency being supplied to the mixers, the intermediate frequencies formed by frequency conversion having mirror symmetry in relation to a sampling frequency or a multiple of the sampling frequency of the analog/digital converter used.
As an alternative or in addition to such a frequency multiplex transmission, the received signals may be digitized within the local coil.